Organic and polymer light-emitting diodes (OLED and PLED) have drawn considerable attention because of their low power consumption, light weight, fast response and wide viewing angle. Charge transport is an important factor with regard to the performance of these devices. For high-performance LED devices, charge injection and transport from both anode and cathode must be balanced off by excitons formed in a light emission layer. Generally, LED devices include three layers sealed between two electrodes, including a hole injection/transport layer (HITL), an electron-emitting layer (EML) and an electron-transporting layer (ETL). Package configurations allow each layer to be optimized individually for charge injection, transport and emission.
Currently developed polymer materials for the hole injection/transport layer (HITL) can mainly be divided into two types based on chemical bonds formed therein, i.e. ionic bond and covalent bond.
Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (abbreviated as PEDOT-PSS hereinafter) as shown in FIG. 1 is one material formed by ionic bonding. Studies on organic materials possessing high hole-injection have been focused on PEDOT-PSS due to its reasonable ionization potential (Ip=−5.2 to −5.3 eV), high conductivity (˜1-10 S/cm) and good hole-injection ability. However, PEDOT-PSS is ionic, acidic and fabricated from water dispersion. Water is relatively more destructive than oxygen for OLEDs and PLEDs. Therefore, PEDOT-PSS is not very stable in the LED architecture. Due to the drawbacks of its material characteristics, currently, PEDOT-PSS cannot be effectively used in the process of large-scale coating.
Further, PEDOT-PSS can be used as a conventional organic transistor element. However, PEDOT has a poor orientational property in an electrode and cannot express a sufficient carrier transport property as an electrode material.
Covalently cross-linked hole injection/transport materials (abbreviated as HITM) leading to the formation of solvent-resistant hole-injection layers have also been extensively studied. Varieties of thermally, photochemically and electrochemically cross-linked materials can overcome the interfacial mixing caused by solution processing. For example, thermosetting polymers formed by covalent bonding as shown in FIG. 2 (Adv. Fun. Mater. 2002. 12 745.; Adv. Mater. 2007, 19, 300.; Adv. Mater. 2009, 21, 1972.) have proper molecular energy levels (Highest Occupied Molecular Orbital (HOMO) Ip=−5.3 eV) and excellent hole injeciotn capacities. Therefore, electronic elements made from the thermosetting polymers have good efficiency. Although the thermosetting polymers formed by covalent bonding do not have the drawbacks of PEDOT-PSS, the property of the thermosetting polymers makes them lack of workability. In addition, the processes for manufacturing electronic elements by the thermosetting polymers are more complex and harsh (for example, it needs a high temperature of over 200° C. to perform thermal curing). Since the cost for manufacturing electronic elements by the thermosetting polymers is high, the polymers currently are suitable for use in small devices of labs and do not have the potential for highly commercial mass production.